Capsaicin pretreatment of guinea pigs in vivo prevents ovalbumin-induced tracheal hyperreactivity in vitro.

The role of substance P-containing sensory nerves in the development of tracheal hyperreactivity to histamine and arecoline was investigated in an allergic model of asthma. Male Hartley-strain guinea pigs were sensitized to ovalbumin (20 mg/kg i.p.) and 14 days later exposed to either saline or ovalbumin (2%) aerosols for 8 consecutive days. One day after the last aerosol exposure the animals were killed and the tracheas were removed. Isotonic muscle shortening in response to increasing doses of histamine and arecoline was measured. Capsaicin (50 mg/kg s.c.) or vehicle pretreatment was performed 7 days prior to sensitization. Tracheas from vehicle-pretreated sensitized and ovalbumin-aerosol exposed animals had increased reactivity to both histamine and arecoline compared to saline-aerosol exposed animals. Capsaicin pretreatment did not alter tracheal reactivity in the saline-aerosol exposed group. Capsaicin pretreatment prevented the increase in tracheal reactivity caused by both agonists in sensitized and ovalbumin-aerosol exposed guinea pigs. These results suggest that capsaicin-sensitive sensory nerves are important for the development of increased tracheal reactivity in an allergic model of asthma.